<<The term "bluestone" in Britain is used in a loose sense to cover all of the "foreign" stones at Stonehenge. One of the most common rocks in the assemblage is known as Preseli Spotted Dolerite—a chemically altered igneous rock containing spots or clusters of secondary minerals replacing plagioclase feldspar. It is a medium grained dark and heavy rock, harder than granite. Preseli bluestone tools, such as axes, have been discovered elsewhere within the British Isles. Many of them appear to have been made in or near Stonehenge, since there are petrographic similarities with some of the spotted dolerites there.

The bluestones at Stonehenge were placed there during the third phase of construction at Stonehenge around 2300 BC. It is assumed that there were about 80 of them originally, but this has never been proven since only 43 remain. The stones are estimated to weigh between 2 and 4 tons each. The majority of them are believed to have been brought from the Preseli Hills, about 250 kilometres away in Wales, either through glaciation (glacial erratic theory) or through humans organizing their transportation. If a glacier transported the stones, then it must have been the Irish Sea Glacier. In such event, one might expect to find other bluestone boulders or slabs near the Stonehenge site, but no such bluestones (apart from fragments) have been found. There is good geological and glaciological evidence that glacier ice did move across Preseli and did reach the Somerset coast. It is uncertain that it reached Salisbury Plain, although a spotted dolerite boulder was found in a long barrow at Heytesbury, which was built long before the stone settings at Stonehenge were installed. One current view is that glacier ice transported the stones as far as Somerset, and that they were transported from there by the builders of Stonehenge.>>

Last edited by neufer on Wed May 02, 2018 5:23 pm, edited 1 time in total.

<<The Irish Sea Glacier was a huge glacier during the Pleistocene Ice Age that, probably on more than one occasion, flowed southwards from its source areas in Scotland and Ireland and across the Isle of Man, Anglesey and Pembrokeshire. It probably reached its maximum extent during the Anglian Glaciation, and it was also extensive during the Late Devensian Glaciation (or Last Glacial Maximum). It was the only clearly defined major glacier in the Irish Sea and flowed about 700 km from its source areas to its southernmost margin. It is sometimes referred to as an “ice stream” since it appears to have been constrained not by ice-free land areas but by highlands which were themselves buried beneath ice. At the time of its maximum extent the glacier extended all the way to the coasts of Somerset and Cornwall, along the south coast of Ireland, and even reached the Isles of Scilly.

It was forced through a relatively narrow constriction in St George's Channel by ice flowing from the Irish Ice Cap (to the west) and the Welsh Ice Cap (to the east). At one point the glacier was probably no more than 80 km wide. Once through this constriction, in the Anglian Glaciation, the ice spread out into a great lobe, in the area now occupied by the Celtic Sea and the approaches to the Bristol Channel. On the eastern flank of the glacier, the evidence of striations, glacial deposits and "erratic trains" shows that the edge of the Irish Sea Glacier was pushed southwards by ice coming from the Welsh ice cap on the Brecon Beacons, so that Irish Sea ice flowed parallel with the coast of South Wales and then came into contact with the English coast around the Somerset Levels. It is not known how far inland this ice extended, but there are scattered glacial deposits in the Bridgwater - Glastonbury district—these may mark the easternmost extent of the glacier. On its western flank the Irish Sea Glacier reached Cork Harbour. The maximum width of this great ice lobe was more than 320 km. "Old" glacial deposits in South wales and in Pembrokeshire are still not accurately dated.>>

Has anyone ever observed a double lunar halo? Perhaps there is not sufficient brightness for the naked eye.

A 44° lunar halo is a second order doubly scattered caustic requiring a thick layer of coherently oriented ice crystals.

https://www.atoptics.co.uk/halo/44pars.htm wrote:
<<The halos of interest in this [David Bainbridge] panorama are the coloured 'smudges' on each side of the sun and twice as far from it along the parhelic circle as the sundogs. At first sight they could simply be brighter fragments of the 46° halo but the enlarged lower view shows that they are not. They are closer to the sun.

They are the fabled 44° parhelia. They were previously photographed during the famous Saskatoon display of December 3, 1970 (during which a Kern arc was also reported but not photographed) and purely visual sightings were occasionally reported since.

The 44° parhelia are rare examples of multiple scattering. In effect they are 'sundogs of sundogs' formed by rays that have already passed through a plate crystal - and would otherwise have formed a sundog - intercepting a second plate crystal and being further deflected. The double deflection has a minimum deviation angle of 44°. Very high plate crystal concentrations are needed. As an alternative explanation, peculiar geometries have been invoked to form the parhelia from only the single crystal scattering that normally produces halos.>>

Has anyone ever observed a double lunar halo? Perhaps there is not sufficient brightness for the naked eye.

A 44° lunar halo is a second order doubly scattered caustic requiring a thick layer of coherently oriented ice crystals.

https://www.atoptics.co.uk/halo/44pars.htm wrote:
<<The halos of interest in this [David Bainbridge] panorama are the coloured 'smudges' on each side of the sun and twice as far from it along the parhelic circle as the sundogs. At first sight they could simply be brighter fragments of the 46° halo but the enlarged lower view shows that they are not. They are closer to the sun.

They are the fabled 44° parhelia. They were previously photographed during the famous Saskatoon display of December 3, 1970 (during which a Kern arc was also reported but not photographed) and purely visual sightings were occasionally reported since.

The 44° parhelia are rare examples of multiple scattering. In effect they are 'sundogs of sundogs' formed by rays that have already passed through a plate crystal - and would otherwise have formed a sundog - intercepting a second plate crystal and being further deflected. The double deflection has a minimum deviation angle of 44°. Very high plate crystal concentrations are needed. As an alternative explanation, peculiar geometries have been invoked to form the parhelia from only the single crystal scattering that normally produces halos.>>